Catechol oxidase, the enzyme that causes browning in plants, catalyzes the oxidation of ortho-diphenols to their quinones with concomitant reduction of O2 to water. Catechol oxidase contains a type 3, dinuclear copper center similar to tyrosinase and hemocyanin. The nature of oxygen activation by metalloenzymes is of great biological interest, but intermediates have been difficult to characterize due to the transient nature of the metal-oxygen species. This study seeks to measure the kinetics and identify intermediates of O2 binding, O-O bond activation and cleavage, and substrate binding in the reaction of catechol oxidase. To accomplish this, the reaction will be poised at several stable and well-characterized steps, and then a novel technique of electron transfer will be will be used to trigger reactions. In this method, electron transfer (ET) either into or out of the active site is achieved using photo-activatible Ru- tethered substrate analogs. ET occurs rapidly (106 s-1) through the covalent link between Ru and the active site ligand, and thus offers a way to regulate the production of intermediates.